power inverter watts for car battery

Power Inverter 12V DC To 110V AC For Your Car Battery

Most all new vehicles (for awhile now) have a built-in DC to AC power inverter. It converts 12 volts DC from your battery /alternator to 110 volts AC which is what’s in your home electrical outlets. This power inverter enables the use of relatively low power AC loads sourced from your vehicle. You might say it’s a source of emergency power. For the sake of preparedness (which is why I’m posting this).

Built-in Power Inverter versus Add-on Higher Power Inverter

My truck (10 years old now) has a 150 watt power inverter built-in. There’s an AC outlet at the console where I can plug in a AC powered device. The problem is as follows… 150 watts is not that much power. At least if you want to do something more serious with it. It’s enough for plugging in battery chargers for handheld tools and such, but you’re not going to run a lot with it. So, what to do?

Here’s a thought… Within the parameters of your vehicle’s battery and alternator specifications, you could physically connect a higher power DC-to-AC power inverter /converter directly to the battery terminal. In turn, you would enable a much higher powered alternative energy source for ‘whatever’. Maybe for use during an emergency (grid down) situation. Run an extension cord into the house to power your chest freezer for an hour – couple times a day to keep it cold (as just one example). At least until the fuel runs out in your vehicle…

Limitations To Size Of The Inverter

So, what are the limitations to achieving a higher power DC to AC power inverter for your vehicle battery?

One limit is the battery itself. The other is the alternator which charges that battery while the vehicle is running.

First, note this caution. Lets say you hook up a high power inverter directly to a 12 volt battery by itself (or a situation whereby the vehicle is OFF). Bear in mind that you won’t be able to operate it for very long (depending on the load). Also be aware that if the vehicle is running (alternator is charging), a load higher than the alternator specifications may eventually cause damage (more on that in a minute…).

Power Inverter Connected To Stand-alone 12 Volt Battery, or Car Battery While Vehicle Not Running

If the vehicle is off, or for any stand-alone 12-volt ‘car battery’, you won’t be able to run a high load for very long. Here’s an example…

Mid-sized batteries for larger cars and SUVs are rated for around 50 amp-hours. Larger vehicles, such as trucks, can have batteries around 75 amp-hours. Typically, car batteries are rated in CA or CCA (Cranking Amps, or Cold Cranking Amps) and not in AH (Amp Hours) like a deep cycle battery which typically have higher AH ratings.

Lets say the car battery is rated for 50 AH (amp hours) (this is not CCA cold cranking amps). What this means is the car battery can technically deliver 50 amps for 1 hour. Or any combination thereof, like 25 amps for 2 hours. Or 1 amp for 50 hours. See how that works?

Don’t Drain Lead Acid Car Battery Below 50%

However! You don’t want to drain a lead acid car battery down below 50% if you can help it. NEVER drain completely, or you’ll damage or kill it. Instead, those consumption numbers exemplified above should be cut in half for the real world. So that would be, 50 amps for half an hour. Or 25 amps for an hour. Hopefully you get the math on that…

Ultimately, that’s not a whole lot of available power consumption over time (depending on what you’re powering) before the battery needs some charging.

Examples: Running a 300 watt load on a 50 Amp-hour rated car battery will drain it to 50% after 1 hour. A 600 watt load, 30 minutes. A 100 watt load, 3 hours.

But what about using the power inverter while the vehicle is running (alternator always charging)? Yes, you could run indefinitely up to a safe margin (amps) that doesn’t exceed your vehicle alternator rating (sort of… keep reading).

Convert Amps to Watts for 12 Volt Battery

Volts x Amps = Watts

Note the high current (amp) values. Appropriate sized (gauge) cables are important.

  • 100 amps = 1200 watts
  • 50 amps = 600 watts
  • 25 amps = 300 watts

Use Power Inverter While Vehicle Is Running

High output alternators (often found on larger vehicles) can deliver between 200 to 350 amps. Smaller cars by comparison, may only use about 100 amps, or less.

Find Your Vehicle Alternator Specifications

How to find out how big your alternator is… The specifications may be in your vehicle paper work, or manual. Maybe you’ll find a part number to reference and research. Do an internet search on your vehicle. Contact your car dealer service department and ask. You may need your VIN for that.

I read (on the internet)(don’t believe everything you read) that you could simply add up all the fuse box values. That would be the magic number for max amps (current). Well, I was contacted by someone who has a 30+ year career in engineering management for one of the “Detroit 3” auto companies. I would like to quote some insight from this person as it has to do with vehicle alternators.

Alternator size is not determined by adding up the size of all the fuses, because all vehicle fused circuits have a “margin” where the fuse is some value above the actual current draw of that circuit.

The amperage output of the alternator is dependent on its “speed” i.e. alternator RPM. A car type alternator generally requires 2200 or more alternator RPM for full output (heavy truck alternators (like a Leece-Neville) are constructed to put out more power at lower RPM –it’s a tradeoff of cost/weight vs output).

Typical car alternator drive ratios (engine pulley size to alternator pulley size) are usually in the neighborhood of 2.5 /3 to 1.  So, at idle — (say 650 engine RPM), an alternator will likely not be putting out its max rated power. (again, drive ratio is a tradeoff of amperage required vs horsepower consumed by the alternator (think fuel economy), and max RPM capability of the engine vs overspeed of the alternator).

At the company I worked for, alternator sizing (output and drive ratio) is “calculated” during the design phase to pass a series of tests. But it is ultimately “proven”, and signed off via the real-world test. The certification test is a “worst case scenario”– it can be actually driven “on road”, or performed in a dynamometer-equipped-test-cell.

Power Inverter Load That’s Greater Than The Alternator Output

It is “possible” to run a load that’s greater than the alternator output — but not desirable for any length of time. The alternator will be putting out its max charge, and the battery will make up the difference between the total load and that provided by the alternator. At max output, the alternator will heat up, and if the vehicle is not moving, there will likely not be enough cooling air flowing under the hood, and damage to the alternator will likely result. Also, at some point. the battery will drain to the point where it cannot provide the needed “make up power”- which will harm the battery.

Higher RPM idle speed would likely correct this for a refer size load,  but is not easily done in modern cars because of the electronic control of the fuel flow (other than someone sitting in the car pushing on the accelerator pedal)

Biggest Size Power Inverter For My Car

Lets say you have a 100 Amp alternator in your vehicle. Using ‘ohms law’ for 12 volts (although charging voltage is higher – but not necessary for this general conversation), that calculates to be 1200 watts of power.

Does that mean you can hook up a 1200 watt power inverter? No, I wouldn’t! The vehicle still needs its own power (though not too terribly much with all accessories turned off). And you definitely do not want to overload and burn out your alternator. I would estimate that 600 watts (half of the alternator rating) would likely be more appropriate in this example. (Do your own due-diligence for your vehicle alternator /charging system ratings).

As for your position of “50%” — I would think that’s a reasonable goal. 

Given that there always is some outlier that will push the envelope– your recommendation is a cautious one, After all, in the business of “prepping”, one needs to recognize if things “go south”– what they have at that moment it likely all they will ever have, and pushing the limit of some item is likely to destroy it — and it’s no longer easily replaceable.

~ “Detroit 3” 30+ year engineer

Direct Connect Power Inverter To Battery

Where to hook up the power inverter? No, you don’t want to use the accessory outlet (formerly known as the cigarette lighter) for its DC power source input. Those are typically fused for ~10 amps. That’s only 120 watts at best.

Rather, you do want to connect your higher power inverter directly to the battery terminals, along with appropriate gauge wire to handle the amps (current). The user manual will most certainly suggest which AWG (American Wire Gauge) size to use – versus total length involved between battery and power inverter.

For temporary power, here’s an example of a power inverter that has alligator battery clamps for direct connection. Obviously that would be temporary. But you could wire this in permanently as well.

BESTEK Pure Sine Wave 500 Watt
(amzn)

Truck Alternators Are Bigger – More Amps For More Watts

If you have a truck, chances are that your alternator may be higher powered than just 100 amps (depending on your options). My F350 has two batteries and a 200 amp alternator. So, that’s 2400 watts. In theory I could connect a power inverter in the range of about 1200 watts and not worry too much about it, so long as everything isn’t on at once. Maybe something like the following…

Ampeak 1200W
(amzn)

Anyway, hopefully you get the idea. It’s simply another way of getting some emergency (or regular use) power if and when you might need it from your vehicle car battery.

Pure Sine Wave Units
(amzn)

By the way, you want pure sine wave. It generally doesn’t cost that much more… Don’t get modified sine wave (in my opinion).

[ Read: Lithium AA Batteries vs Alkaline ]

18 Comments

  1. Ken,
    On our Super-Duty trucks we can add a high output alternator, the Ford chassis has been used for ambulance base for decades, the high output setup will run a 1500w inverter safely if i remember right, dual batteries like with the diesel are a minimum, im actually looking at adding 2 more in the back, there is also aftermarket upgrades, you could go to a Premier power alt that will run higher but intermittent, not sure on specs.

    1. i have had them on my tractors for years, portable power in the woods. handy as hell. all you need is fuel and that may become sketchy in the near future. it’s a short term solution at best, but a solution never the less.
      a pure sine wave doesn’t cost that more?
      what planet do you live on, they are thousands more.

      1. scout, I live on planet earth. Did you see the price of the 500W BESTEK example above? Did you see the price of the 1200W in the example above?

  2. modified sine wave suck for anything digital and/or sensitive.
    Pure sine wave for computers.

    Modified is good for brute force tools like saws, drills, older welding equipment, grinders.
    It also works well with the portable X-Ray machine here.

    I recently built a little power box that has a 12v car jack, 2 usb ports, digital volt meter.
    I velcoo’d my 200W pure sine wave inverter to the top of it, it’s a marine battery box
    that has my old car battery in it.

    1. Horse – I’ve had the opposite experience. All PC type computers made in the past twenty years have global power supplies, they’ll eat anything between 100-240v, 50hz or 60hz, any kind of third-world craptastic power you can throw at it. This may be true of your x-ray machine too. Large power tools however will overheat and lose efficiency with a modified sine or sawtooth waveform.

      1. My exp with two separate laptops and modified sine wave was flickering screens, horribly jumpy mouse pointer and unresponsive functions.
        The above did not happen with pure sine wave.
        I’ve never used inverters for long periods running power tools, just minutes at a time.

  3. Keep the connection between the 12v battery and the inverter as short as possible and use the correct size of cable (see AWG chart) or you may find the inverter will trip due to voltage losses in the cables.

  4. To Ken and TMac: I have 2 Subaru’s now with no vehicle larger than midsized Outback. On the other hand, about the only critical thing I would like to keep running is a large stand-up freezer in my garage with many extension cords. Would one of these devices combined with a well maintained automobile be better than a Honda generator? The last time we had a multiple day outage was the Valentine’s day ice storm of 2021. We cook on gas and have a gas water heater. We almost doubled the small cans of propane we have since that same ice storm. Our power was out for 4 days. I still work in healthcare and I do not know much about electronics or generators. I went to and from work every day during the ice storm. (hospitals have generators)

  5. Ken said, “Your alternator needs to be at least that many total amps in order to power the electrical system under its full load (all fuses added up).”

    Often the sum of the fuses will generally be much more than the alternator’s capacity. At full load those circuits don’t all operate at the value of the fuse. The fuse is to protect wiring and is representative of the current carrying capacity of the circuit and not necessarily how much load is on it. The same goes for your house, you may have a 200 amp service panel and main breaker but might have 30-40 twenty amp circuits in that panel.

    To get the rating for your alternator, it is far more accurate and easier to just look up a replacement alternator for your vehicle on a parts supplier website like Autozone, O’Rielly, Advance, etc. and it will usually give the amperage for the replacements. Look at the cheapest choices and you car’s will likely be equal or a bit higher rating.

  6. Calirefugee – My wife has a Subaru too. I would definitely not use that boxer engine and alternator to keep critical appliances up. You’ll eventually need to do spark plug tube seals and VCG’s, sooner than a normal car, no sense letting it grind away at idle with such pitiful conversion efficiency. Excess loading can lead to premature failure of the alternator too. Honda makes a great product, but we all know where it is really made. You could literally buy 3 “Chinese” inverter generators for the cost of one Honda EU2000i. Start looking at silent power stations too, these are excellent for night time power and just let it parasitically recharge off your generator during the day.

  7. Response to Tmac: Thanks for the speedy and insightful reply. This, once again, shows that just because something can be done does not mean it is a good idea as a long term solution. Thank you for sharing your expertise in an area I no longer dabble in. My experience with generators was limited and dated as well as my mention of a Honda generator. Years ago, in the Mountains, the one generator that worked hard for multiple seasons without fail was…a Honda generator. (circa: 1980’s). I could use some updated advice on silent power stations in addition to brands of generators.
    How did a young seasonal park ranger/college student freeze a deer carcass and keep the parcels of meat frozen without a freezer? I found a store in the small towns near me that rented storage lockers within their walk-in freezer. The fee was small and reasonable considering I was moving around the state 2x per year.

  8. UPDATE: I was contacted by an engineer at one of the “Detroit 3” auto companies. He provided some very helpful input about car alternators. I was also corrected regarding the notion of adding up the fuses as it relates to alternator specs. I have updated the article with this new information.

    1. Yup, hence the rated high output ambulance alternator, you run a 1500w inverter on your trucks stock alternator longer than an hour at full load and you will be ordering a new alternator, high output model will run at full load on idle and be ok, they are designed for it

  9. Safety Warning on power stations >500wh. The cylindrical cells with built-in vents are gone on the big units, now bonded prisma-packs are in everything. In the 1kw/h class I’ve had ten different designs through my lab recently. Eight of them had the lithium packs open to the air and basically glued into polyprop, a disaster waiting to happen. Boeing learned this painful lesson early in the 787 rollout when they replaced all the hydraulics with lithium servos and nearly lost two planes. An emergency AD forced them to enclose all power packs of this size and design in steel enclosures for fire control. And that’s aerospace stuff, now imagine the cheap copy version sitting in your tent or prep shed. So far only two productions samples of 1kw/h power stations have learned the Boeing lesson and encased the battery pack in lap-jointed steel. Please look for this important feature when shopping for power stations.

    1. Wow, good advice – thanks. It’s always somewhat risky when adopting new(ish) technology before bugs (and disasters) are mitigated. And besides that, there’s the extra profit motive by cutting corners, so to speak. It’s all about how the product is spec’d. Looks like they’re simply relying on BMS to do its job without ever a hiccup.

  10. Tmac: Your advice and comments got me off my butt to look for silent power stations. Locally, I have found a dealer for a Goal Zero/Yeti power station in 1000 watt and others in lesser wattage. They were sold out of their 1500 watt yesterday.
    Do you have any advice in regards to the Goal Zero System for use as a home based backup for some critical appliances (stand-up freezer)?
    Thank you for your advice and expertise in advance as you freely share your opinion on this site about what works and what does not. (after testing and observations)

    1. Hi Calirefugee – The Goal Zero Yeti line is a premium product with a premium price tag; they are second to none. I would be a liar if I denied having studied them intimately. There is a lot to like and a few of their ergonomic design cues were – inspirational. However at almost a buck-and-a-half per watt, I say go for it if you can afford it – perhaps more fairly, if money was no object. Another one I’ve looked at in depth is the Eco-Flow Delta. To me it is on-par with the Yeti at a friendlier price point. I suppose the takeaway is to not get married to any one system because newer technology is coming very soon that will render current stuff obsolete. For proof check the price history curve of common Chinese, ‘cheapo’ 1kw power stations. Even with inflation and tariffs the prices have collapsed. They are dumping stock, they know what’s coming. Stick with the brand names for sure though. Lithium technology is very reliable now, but the BMS and charge boards from no-name products are often the weakest point.

Leave a Reply

>>COMMENT POLICY
>>USE OPEN FORUM for Off-Topic conversation

Name* use an alias